Practical application of enzyme sensors has been advanced particularly, as compared with other biosensors. For example, enzyme sensors for measuring glucose, lactic acid, cholesterol, lactose, uric acid, urea, and amino acids are used in medical measurement and in the food industry. An enzyme sensor performs a quantitative analysis of an analyte by reducing an electron acceptor (mediator) with electrons generated by reaction between an enzyme and a target object to be measured (substrate) contained in a sample solution that is a liquid, and electrochemically measuring an oxidation-reduction degree of the electron acceptor. In the case where blood is the sample solution, however, there is the problem that an accurate assay cannot be performed by a biosensor using an oxidation-reduction enzyme, due to the influence of dissolved oxygen. Particularly, an enzyme sensor utilizing GOD (glucose oxidase) is often used in measurement upon a pre-meal insulin injection and evaluation of hypoglycemia. If a higher concentration of glucose than the actual level is indicated due to the influence of dissolved oxygen, this could lead to excessive administration of insulin, and the hypoglycemia would remain undetected. Therefore, a biosensor is in demand that is not influenced by dissolved oxygen even if it is used with respect to a sample that could possibly contain dissolved oxygen, such as blood.
As a technique for avoiding the influence of dissolved oxygen, a method using a third electrode (JP 10 (1998)-282038 A, JP 2000-065778 A), and an enzyme sensor using GDH (glucose dehydrogenase) (JP 2007-163499 A), etc. have been developed. On the other hand, it has been pointed out that new capital investment is needed in the case of the method using a third electrode, and that as to the method using GDH, enzyme costs of GDH are higher than that of GOD, and the method is likely to be influenced by saccharides such as maltose that hinder the measurement.